The present invention relates to a driving circuit for a vacuum fluorescent display and, more particularly, to a driving circuit for supplying power to the filament of a vacuum fluorescent display.
A vacuum fluorescent display is an electron tube which accommodates an anode and a cathode in an evacuated container (envelope) having at least one transparent side surface. The vacuum fluorescent display normally has a triode structure having, between the anode and the cathode, a grid to control movement of electrons emitted by the cathode. In this vacuum fluorescent display, the grid accelerates electrons emitted by the cathode to make them collide against phosphor applied onto the anode. Then, the phosphor emits light, and a desired pattern is displayed.
The cathode normally uses a filament with an electron emission material applied. Power is supplied to the filament to make it generate heat, thereby generating thermoelectrons.
To drive the vacuum fluorescent display, a driving circuit for supplying a filament voltage, a grid voltage, and an anode voltage is necessary.
The filament voltage needs to be a low AC voltage of, e.g., about 5 V. However, the grid voltage and the anode voltage need to be high DC voltages of about 50 V. Normally, the grid and the anode use the same voltage. The grid voltage and the anode voltage will collectively be referred to as a “display voltage” hereinafter.
Conventionally, when supplying the filament voltage and the display voltage to the vacuum fluorescent display, a voltage doubling circuit doubles and rectifies an AC filament voltage to generate a DC display voltage. This arrangement provides partial commonality of the filament voltage power supply and the display voltage power supply.
However, when an AC voltage is doubled and rectified, power loss is large. Additionally, since the voltage doubling circuit becomes hot, the reliability lowers.
A driving circuit which reduces loss by pulse-driving a voltage doubling circuit has been proposed (Japanese Patent Laid-Open Nos. 2003-29711 and 2005-181413).
FIG. 5 shows an example of the arrangement of a driving circuit which pulse-drives a voltage doubling circuit. Referring to FIG. 5, a driving circuit 200 includes a logic power supply 20, a reference oscillator 21, a ½-frequency dividing circuit 22, a filament driver IC 23, and a boost circuit 24.
The logic power supply 20 generates a DC power supply voltage Vcc from an input voltage (DC voltage) Vi.
The reference oscillator 21 includes an inverting amplifier IC, diodes, resistors, and a capacitor, and generates a reference clock signal of about 100 to 200 kHz, as shown in FIG. 6A. The reference clock signal is input to a terminal SEL of the filament driver IC 23. The ½-frequency dividing circuit 22 includes a flip-flop and resistors, and generates an external clock signal by halving the frequency of the reference clock signal, as shown in FIG. 6B. The external clock signal is input to an external clock input terminal EXTCK of the filament driver IC 23.
The filament driver IC 23 switches the input voltage Vi and outputs complimentary differential pulse voltages P1 and P2 from output terminals OUT1 and OUT2 (FIGS. 6C and 6D). The differential pulse voltages P1 and P2 from the filament 6 are supplied to a filament 6 so that an AC filament voltage Ef is applied across the filament 6 (between terminals F1 and F2). When the terminal SEL is at “H” level, an internal clock operation based on an internal oscillator (not shown) of the filament driver IC 23 is performed. When the terminal SEL is at “L” level, an external clock operation based on the external clock signal is performed.
The boost circuit 24 is formed from a voltage doubling circuit including diodes and capacitors, and an emitter follower regulator including a transistor, Zener diodes, resistors, and capacitors. The boost circuit 24 boosts and rectifies the differential pulse voltages P1 and P2 output from the filament driver IC 23, and outputs them as a DC voltage VDD2 for the display voltage.
In the above-described conventional driving circuit, however, when the DC power supply Vi varies, the DC power supply voltage Vcc changes, and the effective voltage supplied to the filament also varies. This causes variations in the amount of electrons emitted by the filament, and degrades the display quality, resulting in, e.g., shorter life of the vacuum fluorescent display or flickering display.